1. Field of the Disclosure
The present disclosure relates to a magnetometric sensor and a current sensor comprising the magnetometric sensor.
2. Description of the Related Art
In the field of such as motor drive technologies of electric vehicles and hybrid cars, relatively large currents are used, and contactless current sensors capable of measuring large currents are in demand. In existing such current sensors, magnetometric sensors detecting induced magnetic fields from currents to be measured are employed. Examples of the magnetism detecting element for the magnetometric sensor include magnetoresistance effect elements such as giant magnetoresistance (GMR) elements.
The GMR element has a layered structure, as the basic structure, composed of a fixed magnetic layer and a free magnetic layer stacked with a nonmagnetic material layer interposed therebetween. In the fixed magnetic layer, the direction of magnetization is fixed in one direction by the exchange coupling bias due to a layered structure of an antiferromagnetic layer and a ferromagnetic layer or by RKKY interaction (indirect exchange interaction) due to a self-pinned structure composed of two ferromagnetic sub-layers disposed with a nonmagnetic intermediate sub-layer interposed therebetween. The free magnetic layer can change the direction of magnetization according to the external magnetic field.
In a current sensor employing a magnetometric sensor comprising a GMR element, an application of an induced magnetic field from the current to be measured to the GMR element changes the direction of magnetization of the free magnetic layer. The electrical resistivity of the GMR element is changed according to the relative angle between this magnetization direction of the free magnetic layer and the magnetization direction of the fixed magnetic layer. Accordingly, the magnetization direction of the free magnetic layer can be detected by measuring this electrical resistivity. Furthermore, the magnitude and the direction of a current providing an induced magnetic field can be determined based on the direction of magnetization detected with the magnetometric sensor.
Incidentally, some electric vehicles and hybrid cars control the driving of the motor based on the value of a current, and some electric vehicles and hybrid cars adjust the process of controlling the battery according to the value of a current flowing into the battery. Accordingly, the current sensor employing a magnetometric sensor is required to increase the measurement accuracy of the magnetometric sensor such that the current value can be precisely detected.
Enhancement of the measurement accuracy of a magnetometric sensor requires, for example, a reduction in offset, a reduction in the variation of output signals, and an improvement in linearity (output linearity). These requirements are preferably achieved by, for example, decreasing the hysteresis of the GMR element of the magnetometric sensor. Specifically, the hysteresis of the GMR element can be decreased by, for example, applying a bias magnetic field to the free magnetic layer to align the magnetization direction of the free magnetic layer even when no induced magnetic field from a current to be measured is applied.
Japanese Unexamined Patent Application Publication No. 2012-185044 discloses an example of a method for applying a bias magnetic field to a free magnetic layer by stacking an antiferromagnetic layer on the free magnetic layer, where the antiferromagnetic layer causes an exchange coupling bias between it and the free magnetic layer to align the magnetization direction of the free magnetic layer in a prescribed direction in a state of permitting variation in magnetization.
The method for causing an exchange coupling bias by means of the antiferromagnetic layer has advantages, such as the uniformness of the bias magnetic field, compared to a method of causing a bias magnetic field by arranging permanent magnetics in the periphery of a GMR element. However, the GMR element stored under a high temperature environment for a long time has a risk of increasing the bias magnetic field due to the exchange coupling bias caused in the free magnetic layer and thereby decreasing the detection sensitivity of the GMR element.
These and other drawbacks exist.